1. Field of the Invention
The invention relates in general to the fabrication of a trench, and more particularly to a method for removing top corner through an additional dry etching process after the formation of trenches.
2. Description of the Related Art
Trench structure has been widely applied for the isolation technique of integrated circuit (IC) fabrication. For example, it is utilized in the isolation technique for dynamic random access memory (DRAM) to have smaller isolation size than the conventional isolation technique of local oxidation. Trench structure is also applied in the structure of the storage capacitor of DRAM. Typically, the technique of etching a trench includes using some particular gas mixture, in corporation with precise control of the hardware.
Dry etching is one of the most common methods of forming a trench. Using plasma but not solution for film etching is characterized by anisotropic etch, which provides much larger vertical etch rate than the horizontal one. It prevents the phenomenon of undercut. The trench structure formed by dry etching includes a straight sidewall and a top corner with a substantially right angle.
FIG. 1A to FIG. 1C show the cross-sectional views of forming a conventional trench. Referring first to FIG. 1A, an oxide layer 11 is first formed in the substrate 10 and then a silicon nitride layer 12 is formed on the oxide layer 11. A photoresist layer is added on the silicon nitride layer 12 for the further process of the anisotropic dry etching. After several trenched 13 are formed, the photoresist layer on the silicon nitride layer is then removed. Referring to FIG. 1B, after forming the trench 13 and removing the silicon nitride layer 12 and the oxide layer 11, an liner oxide layer 14 is grown on the surface of the substrate 10 and the trench 13. Silicon oxide 15 is then filled in the trench 13. The after-etched top corner has an angle of about 90.degree., which causes charge accumulation and leakage current. During the following processes, the metal conductive layer may contact with the top corner, as shown in FIG. 1C. The metal conductive layer 16 may at least bestraddle the substrate 10 and the trench 13. If there is an existence of another structure 17 on the substrate, the metal conductive layer may further bestraddle the structure 17 and the trench 13. The bestaddling of the metal conductive layer and the leakage current due to charge accumulation together result in short circuit, which therefore reduces the reliability and productivity. It is then called Kink effect.